1. Field of the Invention
The present invention relates to a packaging material and, more specifically, to a packaging material such as a closure and a container like bottles and cups, which are capable of stably suppressing the permeation of oxygen through the container walls for extended periods of time.
2. Prior Art
As packaging containers, there have heretofore been used metal cans, glass bottles and a variety of plastic containers accompanied, however, by problems of disintegration of the contents and loss of flavor due to oxygen remaining in the containers and due to oxygen that enters permeating through the container walls.
In the case of the metal cans and glass bottles, quite no oxygen enters permeating through the containers walls, and only oxygen remaining in the containers could cause a problem. In the case of the plastic containers, however, oxygen enters permeating through the container walls to a degree that is no longer negligible arousing a problem from the standpoint of preserving the contents.
In order to prevent this, the plastic containers have been produced having container walls of a multi-layer structure containing at least a layer of a resin having oxygen-blocking property, such as an ethylene/vinyl alcohol copolymer.
A deoxidizing agent has long been used for removing oxygen in the containers. Japanese Examined Patent Publication (Kokoku) No. 1824/1987 discloses an invention using the deoxidizing agent for the container walls, according to which a multi-layer structure for packaging comprises, laminated one upon the other, a layer blended with the deoxidizing agent containing, as a chief agent, a reducing material such as iron powder or the like in the resin having oxygen-permeating property, and a layer having an oxygen gas shut-off property.
Japanese Examined Patent Publication (Kokoku) No. 60826/1992 proposed by the present inventors discloses a multi-layer packaging material comprising a laminated structure of:
an intermediate layer of a resin composition of a gas-barrier thermoplastic resin having an oxygen permeation coefficient at 20xc2x0 C. and 0% RH of not larger than 10xe2x88x9212 ccxc2x7cm/cm2xc2x7secxc2x7cmHg and a water-absorbing amount at 20xc2x0 C. and 100% RH of not smaller than 0.5% blended with an organometal complex of a transition metal; and
layers of a moisture-resistant thermoplastic resin formed on both sides of the intermediate layer.
Japanese Patent No. 2,991,437 teaches a barrier wall for packaging including a composition of a polymer having oxygen-trapping property or including a layer of this composition, wherein the composition traps oxygen as an organic oxidizing component is oxidized with a metallic catalyst, the organic component that can be oxidized being a polyamide and, particularly, a polyamide containing a xylylene group.
It has long been known to use the xylylene-containing polyamide as a gas-barrier resin layer in the multi-layer container, and to blend a thermoplastic resin with a deoxidizing agent to absorb oxygen that enters permeating through the container wall (e.g., see Japanese Examined Patent Publication (Kokoku) No. 1824/1987, Japanese Unexamined Patent Publication (Kokai) No. 278344/1989, International Patent Publication No. 500846/1990).
A method of blending a resin with an oxygen-absorbing agent such as an iron powder to use it as a wall of the packaging material is satisfactory from the standpoint of large oxygen-absorbing ability but cannot be used in the field of packages where transparency is required since it causes the resin to be tinted in a specific hue.
On the other hand, an oxygen-absorbing resin composition containing a catalyst of the type of a transition metal is advantageous since it can be used even for the packaging containers that are substantially transparent accompanied, however, by such a defect that a base resin blended with the catalyst of the type of a transition metal is deteriorated upon the oxidation permitting the permeation of oxygen through the container wall to increase with the passage of time and, further, causing the strength of the container to decrease.
The present inventors have discovered the fact that in a multi-layer packaging material provided with an oxygen-absorbing layer of a thermoplastic resin blended with an oxidizing polymer and a transition metal catalyst, oxygen is selectively absorbed due to the oxidation of the oxidizing polymer and, as a result, the above-mentioned defect is effectively overcome.
Namely, it is an object of the present invention to provide a multi-layer packaging material which has excellent oxygen-absorbing property and is capable of suppressing the permeation of oxygen through the oxygen-absorbing layer for extended periods of time.
Another object of the present invention is to provide a multi-layer packaging material provided with an oxygen-absorbing layer of the type of separated functions, the oxygen-absorbing layer having excellent oxygen-absorbing property and further having excellent oxygen-barrier property.
A further object of the present invention is to provide a multi-layer container made of the above packaging material, exhibiting excellent oxygen-absorbing property and excellent oxygen-barrier property, and withstanding hot filling and preservation over extended periods of time.
According to the present invention, there is provided a packaging material having an oxygen-absorbing layer of a thermoplastic resin which is blended with an organic oxidizing component and with a transition metal catalyst, wherein the thermoplastic resin is not substantially oxidized in the presence of the transition metal catalyst.
In the packaging material of the present invention, it is desired that the thermoplastic resin is a xylylene group-containing polyamide resin having a amino end group (AEG) concentration of not smaller than 40 eq/106 g.
It is further desired that the above-mentioned organic oxidizing component is a polyene and, particularly, a polyene having a functional group on a side chain or at a terminal thereof. In particular, the organic oxidizing component is a polybutadiene or a polyisoprene modified with maleic acid or an anhydride thereof. It is further desired that the organic oxidizing component is dispersed in the thermoplastic resin, and the dispersant has a minimum length of not larger than 400 nm.
Further, it is desired that the above-mentioned transition metal catalyst is an organic salt of cobalt and is contained at a concentration of not smaller than 300 ppm calculated as a metal. It is desired that the packaging material is in the form of a multi-layer container having at least one oxygen-absorbing layer.
In the multi-layer container of the present invention, it is desired that the thermoplastic resin in the oxygen-absorbing layer is a xylylene group-containing polyamide resin having a amino end group concentration of not smaller than 40 eq/106 g, and, particularly, that an exothermic peak of the xylylene group-containing polyamide resin due to the crystallization is not larger than 10 J/g as measured by the DSC, and a half-value width of a maximum value of a diffraction profile at 2 xcex8=0 to 100xc2x0 is not larger than 10xc2x0 as measured relying upon the X-ray diffraction.
It is further desired that a layer of a thermoplastic polyester resin is provided on at least the one side of the oxygen-absorbing layer, that the thermoplastic polyester resin has a crystallinity of from 30 to 55% and that the half-value width of an X-ray diffraction profile thereof is not larger than 15xc2x0.